The present invention relates to fluid coupling devices of the type including both a fluid operating chamber and a fluid reservoir chamber, and valving which controls the quantity of fluid in the operating chamber.
Although the present invention may be used advantageously in fluid coupling devices having various configurations and applications, it is especially advantageous in a coupling device of the type used to drive a radiator cooling fan of an internal combustion engine, and will be described in connection therewith.
Fluid coupling devices ("fan drives") of the viscous shear type have been popular for many years for driving engine cooling fans, primarily because their use results in a substantial saving of engine horsepower. The typical fluid coupling device operates in the engaged, relatively higher speed condition only when cooling is needed, and operates in a disengaged, relatively lower speed condition when little or no cooling is required.
For many years, one of the problems associated with fluid coupling devices used to drive radiator cooling fans has been the phenomenon known as "morning sickness". A typical viscous fan drive includes one or more fill openings, through which viscous fluid flows from the reservoir into the operating chamber, and one or more discharge openings through which fluid is pumped from the operating chamber back into the reservoir. After the fan drive has been operating for a period of time, and the engine is turned off, the fan drive ceases rotation. When the fan drive stops rotating, the orientation of the fill and discharge openings is completely unpredictable, i.e., the fill opening could be in the three o'clock position and the discharge opening could be in the nine o'clock position, in which case there would probably be relatively little "drain back" or "bleed back", i.e., a flow of fluid from the reservoir back into the operating chamber.
However, the fan drive could stop with the discharge opening in the six o'clock position and the fill opening in the twelve o'clock position, in which case the discharge opening would be beneath the level of the fluid in the reservoir. When this situation occurs, it has been conventional for a certain amount of fluid to bleed back from the reservoir through the discharge opening, into the operating chamber. Then, when operation of the fan drive would begin again, after a long period of time (for example, the next morning), with a substantial amount of fluid in the operating chamber, even though the engine is cold and no cooling is required, the fan drive would initially operate in the engaged condition for a period of time until most of the fluid in the operating chamber is pumped back into the reservoir. Unfortunately, such engaged operation results in an undesirable noise of the fan being driven when it is not required. It is this phenomenon, most common and perhaps most objectionable in the morning, which has been referred to as "morning sickness".
Several substantial improvements to overcome the problem of morning sickness have been developed by the assignee of the present invention, such solutions being illustrated and described in U.S. Pat. Nos. 4,312,433 and 5,101,950, both of which are assigned to the assignee of the present invention and are incorporated herein by reference. In both of the cited patents, the general approach to the problem of morning sickness has been to configure the discharge flow path (i.e., the path from the operating chamber, past the pump out means and back to the reservoir) such that fluid in the reservoir would be unable to flow backwards through the discharge path and into the operating chamber, regardless of the rotational orientation of the fan drive during periods of engine shut down.
Unfortunately, it has been observed that, even in the presence of the anti-bleed back solutions in the above-cited patents, a certain amount of bleed back or drain back can still occur through the fill opening whenever, for example, the fan drive stops rotating with the fill opening in the six o'clock position (i.e., below the fluid level) and the discharge opening in the twelve o'clock position. As is well known to those skilled in the art, the fill port needs to be near the radially outer periphery of the reservoir chamber, in order to facilitate filling of the operating chamber, but then, the fill port is even more likely to be well below the fluid level in the reservoir chamber. The bleed back through the fill opening can occur in one of three ways. First, if, as a result of manufacturing inaccuracies or otherwise, the valve arm does not seal tightly over the fill opening, bleed back will occur. Second, if the ambient air temperature around the fan drive is relatively high when the fan drive stops rotating, the valve arm will open (or remain open) and allow fluid to flow from the reservoir into the operating chamber. Third, an electronically controlled fan drive that is of the "fail-safe ON" type will have its valve opened when the electrical power is turned off.
Those skilled in the art have attempted to reduce drain back through the fill opening by improving the capability of the valve arm to seal the fill opening. However, reducing drain back in this manner is difficult and expensive, and even with a perfect seal, there would still be drain back whenever the valve arm would not be covering the fill port at the time the fan drive stopped rotating, as in the second and third scenarios discussed above.